A polyvalent hydroxy compound and an epoxy resin using the polyvalent hydroxy compound have been widely used in a semiconductor encapsulation material or an electronic component such as a print circuit board, electronic component fields, a conductive adhesive agent such as a conductive paste, other adhesive agents, a matrix for a composite material, a coating material, a photoresist material, a developing material, and the like from a viewpoint of providing a cured product having low shrinkage (dimensional stability) at the time of curing, and excellent electrical insulation and chemical resistance. Recently, in such various applications, particularly, in advanced material fields, performance represented as heat resistance or moisture absorption resistance, and low thermal expandability has been required to be further improved. Further, according to the laws and regulations with respect to environmental problems, solder having a high melting point which does not use lead (lead free solder) has been mainly used, and the lead free solder has a working temperature approximately 20° C. to 40° C. higher than that of eutectic solder of the related art, and thus heat resistance which is higher than ever before has been required in the cured product of the epoxy resin.
An epoxy resin having a biphenyl in a molecular skeleton disclosed in PTL 1 has particularly excellent various physical properties such as high heat resistance, low thermal expandability, and moisture resistance. Further, the epoxy resin has crystalline properties at an ordinary temperature, and thus has low viscosity even in a solid resin just like in a liquid resin at the time of being melted. Accordingly, when the epoxy resin is used in a encapsulation material, excellent fluidity is able to be obtained even at the time of being highly filled with silica, and thus the epoxy resin has excellent workability and excellent performance. However, recently, for heat resistance, higher performance has been required, and thus it is necessary that the heat resistance is further enhanced. In order to improve the heat resistance, an increase in crosslinking density, that is, multifunctionalization is effective, and as disclosed in PTL 2 and PTL 3, an epoxy resin having a phenol compound as a skeleton in which phenols are introduced to a biphenyl skeleton as a repeating structure is multifunctionalized, but the molecular weight increases, and thus an increase in a softening point or a melt viscosity occurs. In addition, the multifunctionalization due to an increase in the molecular weight is insufficient to correspond to the recent demand for high heat resistance. In order to improve the heat resistance while maintaining liquid properties or a low melt viscosity, it is necessary that a multifunctional epoxy resin having a low molecular weight is obtained with high purity.